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In this work laser‐welded tube‐tube specimens made of aluminium alloys AlMg3.5Mn and AlSi1MgMn T6 were experimentally tested under constant and variable amplitude loading, under pure axial and pure torsion loading. In order to evaluate the influence on fatigue behaviour of the residual stresses, because of the welding process, some specimens were subjected to postweld heat treatment and then were tested. The numerical analyses, using finite element (FE), were carried out to obtain a reliable estimation of the residual stress in the specimen. The numerical results were in a good agreement with experimental ones obtained by means of hole‐drilling method. Finally, the residual stress distribution was superimposed to stress distribution because of fatigue loads obtained by FE analyses applying local concept, to calculate the stresses in the crack initiation zone and to understand the different types of failure that occurred in as‐welded and relieved specimens. [ABSTRACT FROM AUTHOR]

• Exploring post-processing treatments to improve fatigue resistance of LPBF Ti6Al4V. • High pressure heat treatment produced isotropic α + β macrostructure. • Dry electropolishing and sandblasting reduced inherent surface irregularities. • Tensile properties comparable to forged Ti6Al4V used in automotive field. • Fatigue performance is increased with tailored post-processing treatments sequence. This study investigates the impact of post-processing techniques on the fatigue behaviour of specimens in Ti6Al4V made by laser powder bed fusion (LPBF) for relevant applications, such as motorsport. Despite the LPBF advantages in terms of complex design and weight reduction, challenges such as reduced fatigue performance persist. Therefore, the application of post-processing treatments can play a critical role. In addition to traditional sandblasting (SB), three post-processing conditions were investigated to enhance mechanical properties under quasi-static and fatigue loading conditions: heat treatment (HT) and the innovative High Pressure Heat Treatment (HPHT) along with dry electropolishing (EP) treatment. The results indicate that both the HT and HPHT treatments produced a macrostructure with equiaxial grains composed of fine α + β lamellae. Mechanical analyses demonstrated that the HPHT-SB-EP condition exhibits properties comparable to those of forged Ti6Al4V used in the automotive field, with a good balance between strength and ductility. The EP treatment effectively reduced surface roughness and inherent surface irregularities caused by the LPBF process. The best fatigue results were achieved with the HPHT-SB-EP condition, despite data scattering. An increase in fatigue resistance was observed by using SB at a higher pressure. [ABSTRACT FROM AUTHOR]

Abstract Wendelstein 7-X (W7-X), the world's largest nuclear fusion experiment of modular stellarator type, started operation in 2015 and will be upgraded with a water cooled first wall for steady state operation in 2020. The first wall consists of a CFC armored island divertors, adjacent baffles, heat shields, and stainless steel wall panels. Baffle and heat shield segments consist of graphite tiles, bolted with low pre-stress onto heat sinks of CuCrZr that are in turn brazed onto water cooled steel pipes. Cracks were detected before installation in the baffles in the root of the brazed seam in over 100 locations. Such cracks are attributed to the imposed plastic deformation of the pipes to bring them into the final shape following the complex 3D geometry of the plasma vessel. This paper gives an overview of the experimental and numerical work using finite element method (FEM) and dual boundary element method (DBEM), including sub-modeling to assess the risk of a water leak during operation. Details of the numerical work is published in Giannella et al. (2017), Lepore et al. (2017) and Citarella et al. (2018). First fatigue crack growth experiments were carried out on pipe material and thermal-mechanical crack growth predictions were made with FEM and DBEM. It appeared that the Stress Intensity Factor (SIF) threshold of the ductile steel is only reached when large plastic strains occur, thus violating the field of application of linear elastic fracture mechanics to forecast crack growth. Afterwards, representative brazed pipe samples were manufactured and subjected to initial plastic deformation causing cracks in 11 out of 12 samples. Some samples were tested up to 60,000 bending load cycles. Two out of four samples failed after ∼35,000 cycles. Before and after the test, the shape of the cracks was measured using 3D computer tomography scans. Equivalence between thermal load in W7-X and the mechanical load in the cyclic test was determined with the numerical models to allow for a prediction of the fatigue life in W7X. Additional modeling showed that also plastic zones away from the cracks can limit the fatigue life. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V is an alloy increasingly used in aeronautics due to its high mechanical properties coupled with lightness. An effective technology used to manufacture titanium components with a reduced buy-to-fly ratio is laser beam welding. Previous studies showed that the key factor that rules the mechanical properties and the fatigue life of the joint is its morphology. The aims of this paper were to investigate the influence of the geometrical features of the joints (height of the top and root reinforcement, depth and radius of the underfill, and the valley-valley underfill distance) on their mechanical properties and also to conduct a finite element (FE) analysis on the real geometry of the welded joints. Ti-6Al-4V rolled sheets 3.2 mm thick were welded in butt joint configuration using a laser source and their performance was studied in terms of weld morphology, microstructure, Vickers microhardness and fatigue life. A full factorial plan, designed varying the welding speed and laser power, was carried out. The real geometry and then the joint morphology were studied through an innovative approach: for each specimen, both the total weld face and the total root surface were acquired using a confocal microscope. Finally, through these acquisitions, the clouds of points of the scanned surfaces were used in order to carry out a FE analysis capable of providing a stress concentration factor, K , value for each detected joint. The main results are the realization of a reliable FE model by an experimental agreement and the relationship found amongst the fatigue performances and some noticeable metallurgical and geometrical features, such as the underfill depth and the aspect ratio defined as the ratio between the maximum height of the joint and the valley-valley underfill distance. [ABSTRACT FROM AUTHOR]